Sirtuins (silent mating type information regulation 2 homolog), class III HDACs containing NAD+-dependent protein deacetylase and ADP-ribosyltransferase activities, regulate various metabolic pathways (Denu, 2005; Donmez and Guarente, 2010; Finkel et al., 2009; Haigis and Sinclair, 2010).
Of seven mammalian sirtuins, SIRT1 (NAD-dependent deacetylase sirtuin-1) is the closest homolog of Saccharomyces cerevesiae Sir2 (silent information regulator 2) identified three decades ago (Klar et al., 1979). Loss of SIRT1 causes defective gametogenesis, heart and retinal abnormalities, genomic instability, small body size, and reduced survival in mice (Cheng et al., 2003; McBurney et al., 2003; Wang et al., 2008); and abolishes many beneficial effects of dietary restriction (Chen et al., 2005). Although lifespan extension in Saccharomyces cerevesiae, C. elegans and Drosophila by ectopic Sir2 is still under debate (Burnett et al., 2011; Lombard et al., 2011; Tissenbaum and Guarente, 2001; Viswanathan and Guarente, 2011; Viswanathan et al., 2005), transgenic mice with additional copies of SIRT1 show phenotypes resembling dietary restriction and consistent with improved healthspan (Alcendor et al., 2007; Banks et al., 2008; Bordone et al., 2007; Herranz et al., 2010; Pfluger et al., 2008).
SIRT1 deacetylates a variety of proteins, including KU70, Nbs1, p53, NF-κB, PPARγ, PGC-1α, FOXO, and SUV39H1, and regulates genomic integrity, the inflammatory response, adipogenesis, mitochondrial biogenesis, and stress resistance (Lavu et al., 2008). For example, SIRT1 catalyzes the deacetylation of tumor suppressor protein p53, thus promoting survival by inhibiting p53-mediated apoptosis (Cheng et al., 2003). SIRT1 also directly interacts with PPAR-γ and PGC-1α, thus regulating metabolic response (Picard et al., 2004; Rodgers et al., 2005).
In addition, SIRT1 deacetylates Foxo3a to enhance stress resistance through Foxo3a targets such as MnSOD, catalase, and Gadd45α (Brunet et al., 2004). SIRT1 is highly expressed in embryonic stem cells (ESCs), but its expression is reduced in differentiated cells through a process mediated by miRNAs (Saunders et al., 2010). SIRT1 is required for maintenance of self-renewal of ESCs via modulating p53 cellular distribution and Nanog expression (Han et al., 2008). The hematopoietic differentiation of ESCs is defective and the number and function of hematopoietic progenitor cells decline in SIRT1−/− and SIRT1+/− mice (Lee et al., 2011). When cultured under 5% oxygen, both SIRT1−/− and SIRT1+/− hematopoietic progenitor cells exhibit defective proliferation compared with wild-type cells (Mantel et al., 2008).
SIRT1 is one of the most conserved anti-aging/longevity-promoting proteins across species. Increase in SIRT1 deacetylase activity confers many beneficial effects on various mouse models mimicking human metabolic or degenerative diseases, such as obesity, diabetes, and Alzheimer's Diseases. Therefore, SIRT1-activating compounds could benefit human patients suffering from various metabolic and aging-related degenerative diseases. On the other hand, SIRT1 protein is found upregulated in various human cancers, and inhibition of SIRT1 activity could help in eliminating cancer stem cells (Li et al., 2012).
Increased SIRT1 activity has been documented as beneficial in many disease models and human patients; therefore, it has been widely accepted that SIRT1-activating compounds could provide therapeutic benefits for various metabolic and degenerative diseases (Baur, 2010). On the other hand, the suppressing role for SIRT1 in p53 apoptotic activity suggests tumor-promoting properties of SIRT1 (Luo et al., 2001). Indeed SIRT1 protein has been reported to be elevated in many types of neoplasia, including prostate cancer, acute myeloid leukemia, colon cancer, and various non-melanoma skin cancers (Deng, 2009). It has been recently reported that inhibiting SIRT1 activates p53, thus facilitating the elimination of leukemia stem cells (Li et al., 2012). Therefore, SIRT1-inhibiting compounds confer therapeutic potentials for various human malignancies. Resveratrol, a compound identified in a screen for SIRT1 activators, has been reported to increase lifespan in yeast, worms, and flies, and to enhance healthspan in rodents (Agarwal and Baur, 2011; Baur et al., 2006; Howitz et al., 2003; Milne et al., 2007; Wood et al., 2004). Beneficial effects of resveratrol have been reported on aging-related cataracts, reduced bone density, neurodegenerative diseases, obesity, and diabetes. Resveratrol induces multiple gene expression alterations, mimicking multiple gene expression alterations induced by calorie restriction (CR) (Pearson et al., 2008).
Consumption of RESVIDA®, a resveratrol-containing composition, confers significant metabolic changes similar to that of CR in obese human individuals (Timmers et al., 2011). Studies involving another resveratrol-containing nutraceutical, LONGEVINEX®, revealed that short-term consumption of the nutraceutical can recapitulate the long-term effects of CR (Barger et al., 2008).
A-type nuclear lamins, encoded by the LMNA locus, are type V intermediate filament proteins. The two most prominent A-type lamins, lamin A and C, only differ in the C-terminus where CaaX motif dictates a series of processing events including transient isoprenylation (Rusinol and Sinensky, 2006). A de novo G608G mutation in LMNA promotes alternate splicing, yielding a partially processed prelamin A (also referred to as progerin) that is the predominant cause of Hutchinson-Gilford Progeria Syndrome, a severe form of early-onset premature aging (Eriksson et al., 2003). Mice deficient for Zmpste24, a metalloproteinase responsible for prelamin A maturation, manifest many of the progeroid features resembling Hutchinson-Gilford progeria syndrome (HGPS) patients (Pendas et al., 2002).
The present inventors and other researchers have shown that HGPS skin fibroblasts and mouse embryonic fibroblasts (MEFs) derived from Zmpste24−/− embryos undergo early senescence attributable to genomic instability and hyperactivation of the p53 pathway, and that reduction of the prelamin A level in Zmpste24−/− mice by Lmna heterozygosity ameliorates progeroid phenotypes and significantly extends lifespan (Fong et al., 2004; Liu et al., 2005; Varela et al., 2005). Human cells engineered to express progerin exhibited defective proliferation and premature senescence (Candelario et al., 2008; Kudlow et al., 2008).
Lamin A/C is a major component of the nuclear matrix (NM), a filamentous nucleoskeleton distinct from chromatin and important for maintaining nuclear structure (Fey et al., 1991). Chromatin and other proteins dynamically associate with the NM to regulate various nuclear activities, including replication, gene transcription, DNA repair, and chromatin organization (Blencowe et al., 1994; Kruhlak et al., 2000; Phair and Misteli, 2000). For example, the NM co-purifies with a majority of the nuclear histone deacetylase (HDAC) activity (Downes et al., 2000; Hendzel et al., 1991; Li et al., 1996). One of the hallmarks of HGPS and Zmpste24−/− cells is a misshaped nucleus, which leads to disorganized heterochromatin (Liu et al., 2005; Pendas et al., 2002; Scaffidi and Misteli, 2005) and mislocalized nuclear proteins, such as ATR, SKIP, XPA and Mof (Krishnan et al., 2011; Liu et al., 2005; Liu et al., 2008; Manju et al., 2006; Pendas et al., 2002; Scaffidi and Misteli, 2005, 2006, 2008). Rescue of nuclear shape abnormality by reducing unprocessed prelamin A or progerin from the nuclear envelope via treatment with farnesyl transferase inhibitor (FTI) significantly ameliorates progeroid features in both HGPS cells and mouse models (Capell et al., 2005; Fong et al., 2006; Glynn and Glover, 2005; Toth et al., 2005; Varela et al., 2008).
Alternate splicing events at the wild type LMNA locus can lead to expression of low levels of progerin, which may affect the normal aging process (Scaffidi and Misteli, 2006). An increased number of cells expressing progerin were found during aging in normal individuals (McClintock et al., 2007) and telomere shortening or dysfunction activates progerin production (Cao et al., 2011). These findings suggest that progerin may contribute to the normal aging process (Burtner and Kennedy, 2010), possibly through modulating the activity of proteins implicated in aging.
Over the past several years, calorie restriction (CR)-mimicking properties of resveratrol and SIRT1 protein have attracted considerable efforts in searching for resveratrol mimics and SIRT1 activators. Compounds exhibiting significantly higher SIRT1-activating potential than resveratrol have been identified, and these compounds can elicit similar CR-mimicking beneficial effects as that of resveratrol. In addition, it has been reported that resveratrol specifically enhances SIRT1 activity towards a fluorophore-conjugated synthetic peptide (Ac-Arg-His-Lys-LysAc-AMC) (SEQ ID NO:1) rather than the unmodified one (Borra et al., 2005; Kaeberlein et al., 2005). This observation was later confirmed by other researchers, showing that resveratrol and SRT1720 do not confer any SIRT1 activation towards its full-length native target proteins, including p53 and PGC-1α (Beher et al., 2009; Dai et al., 2010; Pacholec et al., 2010). Therefore, despite various beneficial effects of resveratrol and mimics, the underlying mechanism is still unclear.